1. Field of the Invention
The present invention relates to a distance measuring apparatus, and more particularly, to a passive distance measuring apparatus which does not emit light, such as infrared light, but instead utilizes ambient light to measure a subject distance and which can be used in, for example, a camera.
2. Description of Related Art
Some lens-shutter type cameras are provided with an autofocusing system provided with a passive distance measuring apparatus. This passive distance measuring apparatus includes a pair of image forming lenses (i.e., distance measurement optical system) and a pair of line sensors or light receiving sensors on which subject images are respectively formed through the pair of image forming lenses so as to calculate a subject distance based on triangulation. In cameras of this type, a photographing optical system, a finder optical system and the distance measurement optical system are provided which are independent of each other. In some cameras of this type, the distance measuring apparatus is constructed as a single unit, i.e., a distance measuring unit consisting of the pair of image forming lenses, the pair of line sensors each comprised of an array of a large number of light receiving elements (i.e., photodiodes) on which a plurality of subject images of a common subject are projected, and an arithmetic operating portion for calculating a subject distance based on triangulation in accordance with the data outputted from the pair of line sensors. In the distance measuring unit, the optical axis of the distance measurement optical system does not coincide with either the optical axis of the photographing optical system nor the optical axis of the finder optical system.
In conventional cameras of this type, in the case where the photographing optical system is a zoom lens and the finder optical system is a zoom finder whose magnification varies in accordance with the varied focal length of the zoom lens, the relationship among the finder view formed through the finder optical system, the AF frame which is observed in the finder view and the distance measuring unit will now be described.
When zooming is effected towards the telephoto extremity, a subject image observed in the finder view is magnified due to a variation in the magnification of the zoom finder. However, the distance measuring unit always receives on its pair of line sensors subject images of constant magnification due to the magnification of the pair of image forming lenses of the distance measuring unit being fixed and not varied in accordance with the varied focal length of either the zoom lens or the zoom finder, and furthermore, the size of the AF frame does not change in the finder view. Due to this, on the telephoto extremity side, the focus measuring area indicated by the AF frame, superimposed on a magnified or close-up subject image in the finder view, becomes smaller than the actual focus measuring area determined by the light receiving area of each line sensor in the distance measuring unit.
Accordingly, there is a difference in size between the AF frame in the finder view and the light receiving area of each line sensor in the distance measuring unit. Due to this difference, in conventional cameras of this type, it is often the case that a subject, or subjects, observed out of the AF frame, but close to the AF frame are sometimes erroneously brought into focus through the distance measuring unit as a main subject or subjects, especially when the zoom lens is on the telephoto side. As a result, the main subject is blurry in the resulting picture.
Furthermore, in conventional cameras of the type in which the optical axis of the distance measurement optical system of the distance measuring unit is not aligned with the optical axis of the photographing optical system nor the finder optical system, the optical axis of the distance measurement optical system in the distance measuring unit and the optical axis of the photographing lens are not always arranged to be precisely parallel to each other in an assembling process. If so, that a common subject may not always be captured at the same time by both the distance measuring optical system in the distance measuring unit, and the photographing lens. It is not necessary to adjust the position of the distance measuring unit in the case where a deviation from an optimum arrangement between the optical axis of the distance measurement optical system in the distance measuring unit and the optical axis of the photographing lens is small, i.e., within an acceptable limit. However, if the deviation falls outside the acceptable limit, it is necessary to adjust the distance measuring unit by moving or swinging it so that both the optical axes may be placed parallel to each other to eliminate the deviation. In an adjustment of this kind, the distance measuring unit is moved or swung mechanically relative to the camera body.
After the distance measuring unit has been moved or swung for adjustment, data outputted from the distance measuring unit is checked to find out if it corresponds to predetermined reference data. If the data does not correspond to the reference data the distance measuring unit is readjusted. Therefore, the adjusting operation, in which the distance measuring unit is firstly moved and data is subsequently checked, has to be repeated until such a time that the checked data corresponds to the predetermined reference data, thus resulting in a troublesome, time consuming operation.
Furthermore, in conventional cameras where the camera has a macro photographing mode for close-up photography which the optical axis of the distance measurement optical system of the distance measuring unit is not aligned with the optical axis of the photographing optical system nor the finder optical system, such that the optical axis of the distance measurement optical system of the distance measuring unit deviates from that of the photographing lens by a large distance in left and right directions of the camera, a deviation occurs between the two positions. In other words, the position of a light receiving area on the line sensor on which subject images are projected in regular photography where a distance of subject located on the optical axis of the photographing lens beyond a predetermined distance from the camera is measured, and another position of a light receiving area on the line sensor on which subject images are projected in macro photography for a close-up where a distance of a subject close to the camera is measured within a certain distance range. As a result, the AF frame in the finder view and the light receiving area of each line sensor of the distance measuring unit do not correspond to each other in macro photography, thereby the subject distance cannot be precisely measured.
In a known lens-shutter type camera which has an autofocusing system provided with a distance measuring apparatus including a pair of image forming lenses, a pair of left and right line sensors each comprised of an array of a large number of light receiving elements, used to define a single light receiving area and an arithmetic operating portion for calculating a subject distance based on triangulation in accordance with the data outputted from the pair of line sensors, so that the subject distance can be calculated. However, in the measurement of the subject distance using one light receiving area at each line sensor, as mentioned above, there is only one measurement of the subject distance to be effected, and hence, if no optimum value is obtained by the single measurement or calculation, no focusing can be carried out, thus leading to a missed photographic opportunity.
To solve this problem, it is also known to divide the light receiving elements of each line sensor into a plurality of blocks or groups (i.e., a plurality of light receiving areas), so that the subject distance can be calculated based on sensor data obtained from the pairs of corresponding light receiving areas of the line sensors. However, in this solution, since a plurality of measurements obtained, based on the sensor data supplied from each pair of light receiving areas, are compared to detect the largest value corresponding to the closest distance, so that the focusing can be effected in accordance with the detected largest value, the comparison operation must be carried out for each measurement, contrary to a fast photographing operation.
In a conventional distance measuring apparatus in known cameras, the subject light is split into two halves by a beam splitting optical system. The two halves are converged onto, and received by, respective left and right line sensors. Each line sensor respectively converts the received subject light into electrical image signals which are used for calculation. Namely, for example, a correlativity (degree of coincidence) of the subject image data corresponding to the light receiving areas of the left and right line sensors is evaluated based on the image data at different light receiving areas. When a high degree of coincidence is obtained, position data of the light receiving areas corresponding thereto is detected to calculate a distance between the left and right subject images, based on the position data, and subsequently, the subject distance is calculated, using the calculated distance between the left and right subject images.
However, under conditions having a harmful influence, such as a backlit condition, the amount of light to be received by the left line sensor can be remarkably different from the amount of light to be received by the right line sensor. If this difference occurs, the reference level of image data of the left line sensor (left image data) is different from that of the image data of the right line sensor (right image data), and hence the degree of coincidence decreases. Consequently, it is judged that the subject distance cannot be measured or the subject distance is incorrectly measured. In addition, it is difficult to distinguish the incorrect measurement from that caused by the existence of images of subjects at far and close distances in a light receiving area. Moreover, in some cases, even when a correct measurement has been obtained, the apparatus judges that no distance can be measured.
Furthermore, in a conventional distance measuring apparatus, if the contrast of a subject is low, or if images of subjects at a close distance and at a far distance coexist in a light receiving area, or in the case of a succession of subjects having a repetitive pattern across the light receiving area, no subject distance can be measured. To minimize the occurrence of a subject distance not being able to be measured, a multiple measurement type distance measuring unit is known in which subjects contained in a plurality of light receiving areas can be measured.
However, in a conventional multiple measurement type distance measuring apparatus, the angle of view, i.e., the number of light receiving elements of each line sensor that are used to measure the subject distance for each light receiving area is fixed. If the angle of view is large, i.e., the number of light receiving elements in each light receiving area is large, the subject can be measured in a wide range. Accordingly, the probability that no subject distance can be measured for a subject having a low contrast can be reduced, but the probability that images of subjects at a close distance and at a far distance coexist in a common light receiving area is increased. Conversely, if the angle of view is small, i.e., the number of light receiving elements in each light receiving area is small, the probability that images of subjects at a close distance and at a far distance coexist in a common light receiving area is reduced, but the subject is measured in a narrow range, and accordingly, the probability that no subject distance can be measured for a subject having a low contrast is increased.
Also, in the known multiple measurement type distance measuring apparatus mentioned above, one measurement which meets predetermined requirements is selected from a number of measurements. In an apparatus of this type, the reliability and validity of the measurements are judged relying only upon a single predetermined reference level (judgement level). Namely, if the reference level (judgement level) is high, reliability is increased, but the probability that the measurements do not meet the high reference level increases. Hence, there is likely to be a condition such that no subject distance can be measured. Conversely, if the reference level is low, reliability is reduced, thus resulting in an increase in the occurrence of incorrect measurements.
Moreover, in conventional cameras, it is necessary to measure rays of light at a plurality of light receiving areas in order to judge whether there is a backlit state, through the functions of the camera. To this end, it is necessary to provide a plurality of photosensors which detect rays in the light receiving areas or to use a split-type photosensor.